1) Field of the Invention
The invention relates to a process for treating triazine-containing water of a melamine plant, and to a high-pressure process for preparing melamine from urea.
2) Description of the Related Art
The starting material used for melamine preparation is virtually exclusively urea, which is converted to melamine either in a catalytic low-pressure process or in a high-pressure process without catalyst.
The by-products, degradation products and unconverted starting materials present in the crude melamine have to be removed from the product in the subsequent melamine workup. Since the melamine workup is usually effected in the presence of water, these by-products are obtained as wastewater constituents, for example as constituents of the mother liquor from the melamine crystallization.
The wastewater constituents from the melamine process are mainly cyclic nitrogen compounds in the form of triazines such as melamine or ureidomelamine, or oxoaminotriazines (OATs) such as ammeline or ammelide. In addition, urea or cyanuric acid may also be present. Owing to the aqueous melamine workup in a basic medium, these substances are for the most part present in ionic form. Further ionic constituents which may occur in the melamine wastewater are, for example, carbonates, sodium or ammonium ions.
For the purposes of a maximum melamine yield, it is desirable to recover the melamine present in the water and to recycle it into the melamine process very substantially and selectively. This causes a separation of the melamine from the other wastewater constituents.
The prior art discloses some processes for treating triazine-containing melamine wastewaters, which separate the melamine from the OATs. In some of these processes, the OATs are recovered as solids.
According to WO 01/46159 A2, the mother liquor comprising melamine and OATs is acidified up to pH=7 after the melamine crystallization, which forms an OAT suspension which is then subjected to a tangential filtration. This affords an aqueous melamine solution and an OAT dispersion. While the aqueous melamine solution is recycled into the process, the OATs are separated from the dispersion. One disadvantage in this process is that a stream comprising solids is filtered; as a result, there is the risk of filter blockage.
According to IT 0 128 2369, the wastewater comprising melamine and OAT is treated at high temperature and high pressure, which brings about destruction of the OATs. After the NH3 and CO2 formed in this destruction have been stripped out, the remaining solution is recycled into the melamine plant. In this process, there is the disadvantage that not only the OATs but also the melamine present in the wastewater are destroyed in the thermal treatment, and a satisfactory melamine yield in the overall process accordingly cannot be achieved.
WO 02/100839 A1 describes a process in which a majority of the mother liquors of the melamine crystallization are recycled untreated into the melamine plant; NH3 and OAT are recovered from the smaller portion of the mother liquors. The disadvantage of this process is that more fresh water additionally has to be supplied when the untreated, OAT-containing mother liquors are recycled into the melamine process in order to be able to work up the crude melamine melt to the desired quality.